Institute of Chemistry of Silicates named after I.V. Grebenshchikov

The nature of the phenomenon of negative thermal expansion was studied at the atomic level using the in-situ single-crystal and powder high-temperature X-ray diffraction (HTXRD) of Ho,Tm-doped Yb2Mo3O12 crystals grown by the flux-melt technique. Phase transitions, structure deformation and luminescence were investigated in the temperature range 303-1273 K in air. Under ambient conditions, Ho,Tm-codoped Yb2Mo3O12 has monoclinic crystal structure of Al2W3O12 (P21/c) structure type: a = 16.554 (2), b = 9.859 (1), c = 16.667 (2) Å, β= 107.88 (1)°, V = 2588.6 (5) Å3. A reversible transformation monoclinic ↔ orthorhombic occurs at about 320 K according to the HTXRD data. The orthorhombic structure [Pbcn, a = 13.7388 (2), b = 9.8582 (2), c = 9.9472 (2), V = 1347.25 (4) Å3 at 373 K] shows remarkable negative volumetric thermal expansion up to about 1173 K (average αv = -15 × 10-6 K-1); above this temperature molybdate starts to evaporate. Monoclinic modification in contrast to the orthorhombic one expands only positively. Careful analysis of the orthorhombic crystal structure from non-ambient single-crystal diffraction data showed that the bond lengths in MoO4 and YbO6 polyhedra do not change with temperature; the decrease of structure volume is due to the angular deformation of the framework built from MoO4 and YbO6 polyhedra. This study demonstrates the potential of Yb3+, Ho3+, Tm3+-codoped crystalline systems for developing a highly sensitive ratiometric optical thermometer by exploiting efficient energy transfer, a negative thermal expansion-induced emission stabilization at ∼655 nm within 570 to 800 K, and phonon-activated population of the Tm3+3F3 level for ∼700 nm emission under 980 nm excitation.

Dimethyl sulfoxide (DMSO) is proposed as a solvent at the gelation stage for obtaining transparent aerogels (AGs) based on tetramethyl orthosilicate and 3-aminopropyltrimethoxysilane (APTMS). The transparency of the AGs varies depending on the content of the aminopropyl fragment. The textural characteristics of the AGs are studied. It is shown that the use of DMSO instead of isopropanol, a standard solvent for obtaining the gel, makes it possible to significantly increase the specific surface area of the AGs. Using DMSO as a solvent, transparent AGs based on APTMS modified with residues of perfluorononanoic acid and L-phenylalanine are synthesized. It is shown that it is possible to obtain transparent AGs containing amino complexes of transition metals, namely, copper and palladium.

Abstract The vaporization behavior and thermodynamic properties of the SrO–Al2O3–SiO2 system were investigated by Knudsen effusion mass spectrometry (KEMS) across a composition range of 10–90 mol.% SrO and Al2O3/SiO2 molar ratios of 0.5, 1.0, and 1.5. Experiments were conducted at the temperature 2000 K using a tungsten twin effusion cell to measure partial pressures of gaseous species (Sr, SiO, O2) and derive component activities, Gibbs energies, and excess Gibbs energies. The system exhibited slight negative deviations from ideality, indicative of synergistic interactions among SrO, Al2O3, and SiO2. Compositions with 50–60 mol.% SrO demonstrated minimized evaporation rates and enhanced thermodynamic stability, attributed to the formation of stable Sr–Al–Si–O phases. These findings provide critical insights for designing high‐temperature ceramics for aerospace applications, where thermal resilience and low volatility under extreme conditions are paramount.

Three borosilicates with the general formula of REE3BSi2O10 (REE = Nd, Eu, Gd) were obtained by the high-temperature solid-state synthesis and studied by powder high-temperature X-ray diffraction (HTXRD) in the temperature range from 30 up to 1050°C. The HTXRD study showed that these borosilicates (orthorhombic, Pbca space group) have similar, nearly isotropic thermal expansion in the whole temperature range; the average coefficients of thermal expansion (CTE) were: 〈αa〉 = 9.6, 〈αb〉 = 8.3, 〈αc〉 = 8.7, with αmax – αmin ≤ 1.2 × 10–6°С–1. The average volume CTE insignificantly decreases with increasing the cation size from 27.2 for Eu and 27.0 for Gd down to 25.8 × 10–6°С–1 for Nd compound while the unit cell volume increases with increasing the REE cation radii in the REE3BSi2O10 series.

This article presents the results of determining the crack resistance of quartzoid glasses (QGs) doped with cesium. Crack resistance is calculated on the basis of measured values of microhardness and Young’s modulus. The relationship of crack resistance with the content of cesium oxide (Cs2O 0.76–2.11 wt %) in glass is analyzed. The results of measuring 11B and 29Si NMR spectra are used to interpret the effect of the presence of cesium in glass on the ability of the studied material to resist crack formation.

Abstract Three metamict thorium orthosilicate samples from the syenite pegmatites of the Larvic Plutonic Complex, Norway, were thoroughly examined using Raman spectroscopy, electron probe microanalyses (EPMA), electron back-scatter diffraction (EBSD) and differential scanning calorimetry (DSC). Their thermal evolution upon heating was investigated using in situ powder X-ray diffraction (HTXRD) in the range of 25–1200°C. One of the samples is a colour-zoned metamict thorium silicate with a preserved tetragonal shape. The zonation is due to the increasing hydration and element distribution. The EBSD indicates that the ratio of huttonite to thorite after the crystallisation significantly varies from zone to zone within the same sample. The crystallisation of thorite starts in the range of 420–480°C (lower than reported previously for mineral samples), while the emergence of huttonite peaks in HTXRD patterns occurs at 870–930°C. In contrast to huttonite, no thorite crystallisation peak is observed in the DSC curve. A wide temperature range is observed where both thorite and huttonite can coexist. Several fluorite-type phases form upon heating. Thorianite exists in the range of 810–1140°C. After the cooling, except for huttonite and thorite, the minor crystallised phases vary and may be represented by Ca–Th oxides and rhombohedral CaUO 4 . The thermal expansion of the crystalline huttonite and thorite was determined as $\overline\alpha$ V = 20.66 × 10 –6 deg –1 for huttonite and $\overline\alpha$ V = 12.54 × 10 –6 deg –1 for thorite in the temperature range 25–1200°C. These findings contribute to a more in-depth understanding of the behaviour of thorium orthosilicates with complex compositions, both metamict and crystalline, at elevated temperatures. They have potential applications in mineralogy, nuclear chemistry and high-level waste management.

Thermal expansion of samarium oxoborogermanate Sm14(GeO4)2(BO3)6O8 is studied by the method of high temperature powder X-ray diffraction in the temperature range of 30–1200°C. The coefficients of thermal expansion are calculated: α11 = 9.59(12), αc = 7.56(13), αV = 26.74(30) × 10–6°C–1 at 30°C; and αa = 14.44(12), αc = 10.74(13), αV = 39.61(28) × 10–6 °C–1 at 1200°C. A structural interpretation of the anisotropy of thermal expansion is carried out. With increasing temperature, the degree of anisotropy remains practically unchanged, the structure expands minimally along the c axis, and maximally in the ab plane, perpendicular to the preferred orientation of the BO3 triangles in the crystal structure. The melting point of Sm14(GeO4)2(BO3)6O8 is clarified.